1. Field of the Invention
This patent relates to the field of addressable memory cells and in particular to those used for illumination and display devices. The invention provides to a memory cell utilizing breakover conduction for storing a charge corresponding to an illumination state while providing a controlled and limited current through an emissive element to provide illumination. An organic light emitting memory array incorporates an array of memory cells fabricated with organic semi-conductive materials.
2. Description of the Related Art
Dielectric barrier discharge devices such as plasma display panels (PDPs) set a wall charge on a dielectric surface according to a display image. During operation, an initialization period initializes the wall charge of each illumination cell; during an addressing period, rows of illumination cells are selected and wall charges are set according to display data; and during a sustain period, set wall charges are maintained while producing illumination. These devices have the desirable feature of capacitive memory at each discharge location and capacitively controlled illumination power. Such devices are costly to manufacture and have a need for improved luminous efficacy and other reductions in power consumption.
A PDP, being a gas discharge device, contains a dischargeable gas which exhibits a breakover characteristic. When de-energized, the dischargeable gas is capacitive, having high impedance. When a voltage is applied across the dischargeable gas, in excess of the breakdown voltage of the dischargeable gas, the dischargeable gas becomes energized and therefore conductive only while a voltage is maintained across the dischargeable gas. As current flows through the dischargeable gas, electrical charges are transferred between dielectric surfaces. Once the dielectric surfaces are charged and the voltage across the gas is reduced to zero, the dischargeable gas de-energizes and returns to the high impedance state.
Active matrix organic light emitting diode (AMOLED) displays offer improved luminous efficacy over PDP's but require a substantially constant current flow through each illumination cell to provide illumination in proportion to a display image. Electrode resistance affects the brightness uniformity of these devices and limits their size. To enable large area displays, methods are needed to control the current while maintaining high operating efficiency.
Non-gaseous, i.e. semiconductor based, breakover conduction devices are commonly used for activating AC switches such as TRIACs. These breakover conduction devices exhibit high impedance, i.e. a capacitive characteristic, prior to the application of a voltage thereacross greater than the device's predetermined breakover voltage. Once the devices breakover voltage is exceeded, the device switches to a low impedance state while current through the device is maintained. An AC diode (DIAC) is a simple two terminal bidirectional breakover conduction device. Once the voltage applied (positive or negative) across a DIAC exceeds its breakover voltage, it turns on, self-latching into a conductive state until the current flowing through the device decreases below a minimum holding current. Such devices do not have memory nor an ability of limit current flow.